Wireless communication systems are widely deployed to provide various types of communications such as voice and data. One such system is wideband code division multiple access WCDMA, which has been adopted in various competing wireless communication standards, for example 3rd generation partnership project 3GPP and 3GPP2.
To overcome data corruption that can occur during RF transmission the different wireless communication standards typically include some form of channel coding. For example, WCDMA standards typically require that a WCDMA receiver decode a mixture of turbo encoded and viterbi encoded data streams, where viterbi encoded channels are typically used for time critical data.
In particular the 3GPP standard has specified a high speed data packed access HSDPA sub-system that has two physical channels in the downlink direction; a data channel and a control channel in which turbo coding is used to encode the data channel and convolutional coding is used to encode the control channel.
A turbo encoder uses a first convolutional encoder to encode information bits (i.e. systematic bits) within a packet to generate a first sequence of parity bits (i.e. parity 1 bits) in parallel to the interleaver shuffling the information bits, where the shuffled information bits are encoded by a second encoder to generate a second sequence of parity bits (i.e. parity 2 bits). The information bits and the parity bits in the first and second sequence are then modulated and transmitted to a receiver.
The information bits and the first and second sequence of parity bits are received by a receiver and decoded by a turbo decoder.
For turbo encoded data a turbo decoder initially stores the received information bits and the parity bits in the first and second sequence in a buffer. Initially, the information bits and the first sequence of parity bits from the first convolutional encoder are retrieved from the buffer and decoded by a first decoder (i.e. a first soft in soft out SISO decoder), using a log-MAP algorithm, to provide ‘extrinsic’ information (i.e. a-posteriori data) indicative of adjustments in the confidence in the detected values for the information bits. Intermediate results (i.e. a-priori) that include the extrinsic information from the first decoder are then stored in the buffer in an interleaved order matching the code interleaving used at the transmitter.
The intermediate results, the information bits and the second sequence of parity bits from the second encoder are retrieved from the buffer and decoded by a second decoder (i.e. a second SISO decoder) to provide extrinsic information indicative of further adjustments in the confidence in the detected values for the information bits. Intermediate results that comprise the extrinsic information from the second decoder (i.e. a second SISO decoder) are then stored in the buffer in a deinterleaved order complementary to the code interleaving performed at the transmitter. The intermediate results are used in a next decoding iteration performed by the turbo decoder. The turbo decoder performs a predetermined number of decoding iterations before producing a decision on the value of the decoded information bit.
A viterbi decoder is used to decode convolutional encoded data using a viterbi algorithm.
The basic transmission unit in a HSDPA sub-system is called a time transmission interval TTI where each TTI spans 2ms and contains three identical time periods called slots.
As shown in figure 1, for each TTI transmitted in the data channel 100 there is a corresponding TTI in the control channel 200 that starts 2 slots before the beginning of the associated data channel TTI.
The control data is divided into two parts. The first part 102, which contains information required for the demodulation of the corresponding data channel TTI, is transmitted in the first slot of the control channel TTI. The second part 103, which contains data required for the channel decoding of the corresponding data channel TTI, is transmitted in the second and third slots of the control channel TTI.
There is a period of one slot to decode the first part of the control channel before the decoded data is required for the demodulation of the data channel. Similarly, there is a two slot period to decode the second part of the control channel before the decoded data is required for decoding of the data channel. This arrangement results in severe timing restrictions on the decoding of the control channel part.
One solution to this problem has been the use of a separate turbo decoder for decoding the turbo encoded channels and a separate viterbi decoder for decoding the convolutional encoded channels; however this results in increased cost and size of a receiver.
It is desirable to improve this situation.